Drilling for oil and gas off-shore is completed from one of two types of drilling rigs: rigs that are supported by the sea floor (such as fixed drilling rigs or jack-up drilling rigs) or rigs that float on the surface of the water (such as drill ships or semi-submersible drilling rigs). Although drilling operations conducted from these two types of drilling rigs are similar, at least one major difference exists: drill ships or semi-submersible drilling rigs move with the waves of the sea, while fixed or jack-up drilling rigs remain fixed to the sea floor.
The movement of drill ships or semi-submersible drilling rigs with the waves of the sea presents a unique problem in drilling with these types of rigs. First, in any drilling operation conducted from floating rigs, compensation for the rig's tendency to heave—that is move up and down with the waves—must be accounted for. In particular, as the floating rig moves up and down, the drill string and drill bit extending below the rig will also move up and down. For a drill bit to perform as efficiently as possible, the desired or optimum weight on the drill bit—i.e., the downward force applied to the bit—must be kept as constant as possible. Heave, however, removes weight from the drill bit as the ship or rig rides to the crest of a wave, and puts weight back on the drill bit as the ship or rig rides down into the trough between waves. This fluctuation in the force applied on the drill bit severely hinders an operator's ability to drill the well bore. See Ron Baker, A Primer of Offshore Operations, pgs. 55–63 (Univ. of Texas Petroleum Extension Servs., 2nd Ed., 1985).
Perhaps more importantly, heave creates the potential for blowouts due to a potential fracturing or breaking of the production tubing during testing, workover, or completion operations. Specifically, once the well bore has been drilled, the oil and gas reserves are brought up to the floating rig through production tubing that runs from the rig to the producing zones of the well bore—typically thousands of feet below the sea floor. The string of production tubing consists of dozens, if not hundreds, of joints of tubing—typically approximately 30 feet in length each—connected together. The production tubing is supported by and is kept in tension by the drill hook and drawworks on the drilling rig to keep the string from buckling.
The production tubing is typically held in place within the well bore by one or more production packers. Because the production tubing is held in place within the well bore, any rise of the floating drilling rig due to heave will increase the tension on the production tubing string and could cause the string to fracture or break. A fracturing or breaking of the production tubing string would allow the oil or gas within the tubing to leak, creating the potential for a blowout.
To account for the problems associated with heave, floating drilling rigs are equipped with a heave compensation system. The heave compensation system is typically in the form of an active heave drawworks system or a system that is an integral part of the drilling derrick or mounted directly on an extension of the traveling block. When functioning properly, these primary heave compensation systems are capable of protecting against the effects of heave. However, prior art floating drilling rigs are generally not equipped with a back-up, or secondary, heave compensation system that operates in the event the primary heave compensation system is not functioning properly or becomes inoperative. In such a situation, the floating drilling rig will have no way to compensate for heave.
One possible reason why back-up heave compensation systems have not previously been utilized on drill ships or semi-submersible drilling rigs is the limited space available on the derrick of such floating rigs. Further, the possible locations on the drilling derrick or drill floor that a back-up heave compensation system can be placed is limited by the necessity to allow access to the production tree on the drilling rig. Such access is necessary to conduct numerous drilling operations, including the potential for conducting coiled-tubing operations. These space and placement limitations are likely a significant part of the reason why prior art floating drilling rigs have heretofore not been equipped with a back-up heave compensation system.
Accordingly, what is needed is a heave compensation system that acts as a back-up system to the primary heave compensation system and that is compact enough to fit in the limited space available on a floating drilling rig. It is, therefore, an object of the present invention to provide a heave compensation apparatus that is normally static when the primary heave compensation system is operative, but becomes operative if the primary heave compensation system malfunctions or becomes inoperative. It is a further object of the present invention to provide a back-up heave compensation system that is compact and self-contained such that it can be installed in the limited space available on a floating drilling rig. Those and other objectives will become apparent to those of skill in the art from a review of the specification below.